EVOLUTION-MANAGER

Edit File: shape_tree.h

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
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http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
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==============================================================================*/

#ifndef TENSORFLOW_COMPILER_XLA_SHAPE_TREE_H_
#define TENSORFLOW_COMPILER_XLA_SHAPE_TREE_H_

#include <functional>
#include <iterator>
#include <memory>
#include <vector>

#include "absl/memory/memory.h"
#include "absl/types/optional.h"
#include "absl/types/span.h"
#include "tensorflow/compiler/xla/layout_util.h"
#include "tensorflow/compiler/xla/shape_util.h"
#include "tensorflow/compiler/xla/status_macros.h"
#include "tensorflow/compiler/xla/xla_data.pb.h"
#include "tensorflow/core/lib/core/errors.h"
#include "tensorflow/core/lib/core/status.h"
#include "tensorflow/core/lib/gtl/iterator_range.h"
#include "tensorflow/core/platform/logging.h"
#include "tensorflow/core/platform/types.h"

namespace xla {

namespace internal {

// Internal representation of each node in a ShapeTree.
template <typename T>
struct ShapeTreeNode {
  // Data corresponding to this node.
  std::pair<ShapeIndex, T> data;

bool is_leaf = true;

explicit ShapeTreeNode(ShapeIndex index)
      : ShapeTreeNode(std::move(index), T()) {}
  ShapeTreeNode(ShapeIndex index, T data)
      : data(std::move(index), std::move(data)) {}
};

// Internal representation of an index table entry.
struct IndexTableEntry {
  // Index of the node in the ShapeTreeNode vector.
  uint32 index;
  // Index of the first child in a IndexTableEntry vector. In the index
  // table all children entries for a given node will be placed next to each
  // other. This allows us to use a single field to index them.
  uint32 children_start;
#ifndef NDEBUG
  // Number of children, used for bounds checking.
  uint32 children_count;
#endif
};

}  // namespace internal

template <typename ContainerType, typename IteratorType, typename ValueType>
class ShapeTreeIterator;
template <typename ContainerType, typename IteratorType, typename ValueType>
class ShapeTreeLeafIterator;

// A ShapeTree<T> is a recursive data structure which mirrors the structure of a
// XLA shape and holds a value of type T for each subshape (i.e. tuple or array)
// in the shape. For array shapes, a ShapeTree trivially holds a single value of
// type T.
//
// For tuple shapes which can be an arbitrary tree with arrays at the leaves, a
// ShapeTree is an identically structured tree with data elements of type T at
// every node. I.e. the root is a tuple by definition, all interior nodes are
// also tuples, and all leaves are arrays.
//
// Like the Shape data structure, this is a tree and tuple elements cannot be
// duplicated. That is, every distinct ShapeIndex in the Shape has a unique T
// object.
//
// Normally a ShapeTree owns its Shape, but for efficiency reasons, sometimes
// it's helpful not to copy a Shape just to make a ShapeTree.  In these cases,
// you can pass a Shape* instead of a Shape& to the ShapeTree constructor.  It's
// then up to you to ensure that the pointed-to Shape doesn't die or mutate
// before its ShapeTree goes away.
template <typename T>
class ShapeTree {
 public:
  using Node = internal::ShapeTreeNode<T>;
  using Index = internal::IndexTableEntry;

// Default constructor creates a tree with a nil shape (i.e. an empty tuple).
  ShapeTree() : ShapeTree(ShapeUtil::MakeNil()) {}

// Create ShapeTree with the given shape, and default-constructed T values for
  // all nodes.
  //
  // The version that takes a pointer may be cheaper because it doesn't require
  // any Shape copies, but then it's up to you to ensure that the pointer stays
  // alive longer than this ShapeTree.
  explicit ShapeTree(Shape shape);
  explicit ShapeTree(const Shape* shape);
  explicit ShapeTree(const std::shared_ptr<Shape>& shape);

// Create ShapeTree with the given shape, and init_value for all nodes.
  ShapeTree(Shape shape, const T& init_value);
  ShapeTree(const Shape* shape, const T& init_value);
  ShapeTree(const std::shared_ptr<Shape>& shape, const T& init_value);

// Returns the data element associated with the array in the shape at the
  // given index (see ShapeUtil::GetSubshape for how indexes are defined).
  const T& element(ShapeIndexView index) const;
  T* mutable_element(ShapeIndexView index);

// Return the shape represented with this ShapeTree.
  const Shape& shape() const { return *shape_; }

// A ShapeTree object can own the underlying Shape pointer (via the
  // shape_storage_ member), or can point to a Shape object owned by the caller.
  // This API replaces the underlying Shape object to the one supplied by the
  // caller, whom must ensure the object remain valid for the whole lifetime of
  // this ShapeTree object, and also that the Shape is consistent with it.
  void replace_shape_ptr(const Shape* shape) {
    if (shape_storage_ != nullptr) {
      DCHECK_EQ(*shape, *shape_storage_);
      shape_storage_ = nullptr;
    }
    shape_ = shape;
  }

// Returns true if the node at the given index is a leaf node (an array
  // shape).
  bool IsLeaf(ShapeIndexView index) const { return Lookup(index)->is_leaf; }

ShapeTree(const ShapeTree&) = default;
  ShapeTree& operator=(const ShapeTree&) = default;
  ShapeTree(ShapeTree&&) = default;
  ShapeTree& operator=(ShapeTree&& other) = default;

// iterator implements a bidirectional_iterator with
  //  value_type = std::pair<ShapeIndex, T>.
  //
  // The iteration order is guaranteed to be a pre-order walk of the ShapeTree.
  using iterator =
      ShapeTreeIterator<std::vector<Node>, typename std::vector<Node>::iterator,
                        std::pair<ShapeIndex, T>>;
  using const_iterator =
      ShapeTreeIterator<const std::vector<Node>,
                        typename std::vector<Node>::const_iterator,
                        const std::pair<ShapeIndex, T>>;
  using reverse_iterator = std::reverse_iterator<iterator>;
  using const_reverse_iterator = std::reverse_iterator<const_iterator>;

using leaf_iterator =
      ShapeTreeLeafIterator<std::vector<Node>,
                            typename std::vector<Node>::iterator,
                            std::pair<ShapeIndex, T>>;
  using const_leaf_iterator =
      ShapeTreeLeafIterator<const std::vector<Node>,
                            typename std::vector<Node>::const_iterator,
                            const std::pair<ShapeIndex, T>>;
  using reverse_leaf_iterator = std::reverse_iterator<leaf_iterator>;
  using const_reverse_leaf_iterator =
      std::reverse_iterator<const_leaf_iterator>;

// begin/end for iterating over all nodes.
  iterator begin() { return iterator(&nodes_, nodes_.begin()); }
  iterator end() { return iterator(&nodes_, nodes_.end()); }
  const_iterator begin() const {
    return const_iterator(&nodes_, nodes_.begin());
  }
  const_iterator end() const { return const_iterator(&nodes_, nodes_.end()); }

// rbegin/rend for iterating over all nodes in reverse.
  reverse_iterator rbegin() { return reverse_iterator(end()); }
  reverse_iterator rend() { return reverse_iterator(begin()); }
  const_reverse_iterator rbegin() const {
    return const_reverse_iterator(end());
  }
  const_reverse_iterator rend() const {
    return const_reverse_iterator(begin());
  }

// leaf_begin()/leaf_end() iterates over all leaf nodes (nodes with no
  // children).
  leaf_iterator leaf_begin() { return leaf_iterator(&nodes_, nodes_.begin()); }
  leaf_iterator leaf_end() { return leaf_iterator(&nodes_, nodes_.end()); }
  const_leaf_iterator leaf_begin() const {
    return const_leaf_iterator(&nodes_, nodes_.begin());
  }
  const_leaf_iterator leaf_end() const {
    return const_leaf_iterator(&nodes_, nodes_.end());
  }
  // range-based iterator for leaf_begin()/leaf_end().
  tensorflow::gtl::iterator_range<leaf_iterator> leaves() {
    return tensorflow::gtl::make_range(leaf_begin(), leaf_end());
  }
  tensorflow::gtl::iterator_range<const_leaf_iterator> leaves() const {
    return tensorflow::gtl::make_range(leaf_begin(), leaf_end());
  }

reverse_leaf_iterator leaf_rbegin() {
    return reverse_leaf_iterator(leaf_end());
  }
  reverse_leaf_iterator leaf_rend() {
    return reverse_leaf_iterator(leaf_begin());
  }
  const_reverse_leaf_iterator leaf_rbegin() const {
    return const_reverse_leaf_iterator(leaf_end());
  }
  const_reverse_leaf_iterator leaf_rend() const {
    return const_reverse_leaf_iterator(leaf_begin());
  }

// Returns an iterator pointing to the given ShapeIndex.
  // REQUIRES: index must exist in the ShapeTree.
  iterator find(ShapeIndexView index) {
    Node* element = Lookup(index);
    auto element_iter = nodes_.begin() + (element - &nodes_[0]);
    return iterator(&nodes_, element_iter);
  }
  const_iterator find(ShapeIndexView index) const {
    const Node* element = Lookup(index);
    auto element_iter = nodes_.cbegin() + (element - &nodes_[0]);
    return const_iterator(&nodes_, element_iter);
  }

// Returns the number of leaf nodes in the tree.
  int64 leaf_count() const { return std::distance(leaf_begin(), leaf_end()); }

// Recursively traverses the shape and calls the given function at each
  // element. The function has the following arguments:
  //
  //   Fn :    A callable of type void(const ShapeIndex& index, const T& data)
  //           (or compatible).
  //   index : the index of the element in the shape. See ShapeUtil::GetSubshape
  //           for definition of index.
  //   data : The data value at this element.
  template <typename Fn>
  void ForEachElement(const Fn& func) const;

// Like ForEachElement, but the callable has type
  //
  //   void (const ShapeIndex& index, T* data).
  //
  template <typename Fn>
  void ForEachMutableElement(const Fn& func);

// Like ForEach(Mutable)Element, but the callable returns a Status instead of
  // void.  The first non-OK return value is returned by the ForEach* function.
  template <typename Fn>
  Status ForEachElementWithStatus(const Fn& func) const;
  template <typename Fn>
  Status ForEachMutableElementWithStatus(const Fn& func);

// Maps each element to generate a new tree with the same shape.
  template <typename U>
  ShapeTree<U> Map(const std::function<U(const T&)>& func) {
    ShapeTree<U> result(shape_storage_);
    ForEachElement([&](const ShapeIndex& index, const T& t) {
      *result.mutable_element(index) = func(t);
    });
    return result;
  }

template <typename U>
  ShapeTree<U> Map(const std::function<U(T*)>& func) {
    ShapeTree<U> result(shape_storage_);
    ForEachMutableElement([&](const ShapeIndex& index, T* t) {
      *result.mutable_element(index) = func(t);
    });
    return result;
  }

// Copy the subtree of values from 'other' rooted at ShapeIndex
  // 'source_base_index' into the subtree of value in this ShapeTree rooted at
  // 'target_base_index'.
  //
  // Precondition: The subshape of other.shape() at index source_base_index must
  // be compatible with the subshape of shape() at index target_base_index.
  void CopySubtreeFrom(const ShapeTree<T>& other,
                       const ShapeIndex& source_base_index,
                       const ShapeIndex& target_base_index);

StatusOr<ShapeTree<T>> SubShapeTree(const ShapeIndex& index) const;

bool operator==(const ShapeTree<T>& other) const;
  bool operator!=(const ShapeTree<T>& other) const { return !(*this == other); }

private:
  // Initialize node->children based on 'shape'. All children are assigned the
  // the given 'init_value'.
  void InitChildren(const Shape& shape, const T& init_value, Node* node,
                    Index* index);

// Initialize node->children based on 'shape'. All children have
  // default-constructed data values.
  void InitChildren(const Shape& shape, Node* node, Index* index);

// Returns the number of subshapes, including interior nodes, in shape.
  int64 CountSubshapes(const Shape& shape);

// Helpers for traversing the shape via ForEachElement. The helpers
  // recursively traverse the subtree rooted at "index" (defined as in
  // ShapeUtil::GetSubshape).
  template <typename Fn>
  static Status ForEachHelper(const Fn& func, const std::vector<Node>& nodes);
  template <typename Fn>
  static Status ForEachMutableHelper(const Fn& func, std::vector<Node>* nodes);

// Return the tree node at the given index.
  Node* Lookup(ShapeIndexView index);
  const Node* Lookup(ShapeIndexView index) const;

// The nodes in this shape tree.
  std::vector<Node> nodes_;

// Index table for node lookups.
  std::vector<Index> index_table_;

// If we own our Shape, this field contains it, and shape_ is a pointer into
  // here.  Otherwise if we don't own our shape, this is nullptr.
  std::shared_ptr<Shape> shape_storage_;

// The XLA shape mirrored in this ShapeTree.  This is either
  // shape_storage_.get() or the Shape pointer passed to our constructor.
  const Shape* shape_;
};

// Internal iterator that performs a pre-order walk. This is cheap to copy.
// The iterator value_type is equivalent to a
// std::pair<ShapeIndex,T>&, similar to std::map.
template <typename ContainerType, typename IteratorType, typename ValueType>
class ShapeTreeIterator
    : public std::iterator<std::bidirectional_iterator_tag, ValueType> {
 public:
  ShapeTreeIterator(ContainerType* nodes, IteratorType node)
      : nodes_(nodes), node_(std::move(node)) {}

ShapeTreeIterator& operator++() {
    ++node_;
    return *this;
  }
  ShapeTreeIterator operator++(int) {
    auto i = *this;
    ++(*this);
    return i;
  }

ShapeTreeIterator& operator--() {
    --node_;
    return *this;
  }
  ShapeTreeIterator operator--(int) {
    auto i = *this;
    --(*this);
    return i;
  }

bool operator==(const ShapeTreeIterator& other) const {
    return node_ == other.node_;
  }
  bool operator!=(const ShapeTreeIterator& other) const {
    return node_ != other.node_;
  }
  ValueType& operator*() const { return node_->data; }
  ValueType* operator->() const { return &node_->data; }

private:
  ContainerType* nodes_;
  IteratorType node_;
};

// Internal iterator that performs a pre-order walk of the leaves. This is cheap
// to copy. The iterator value_type is equivalent to a std::pair<ShapeIndex,T>&,
// similar to std::map.
template <typename ContainerType, typename IteratorType, typename ValueType>
class ShapeTreeLeafIterator
    : public std::iterator<std::bidirectional_iterator_tag, ValueType> {
 public:
  ShapeTreeLeafIterator(ContainerType* nodes, IteratorType node)
      : nodes_(nodes), node_(std::move(node)) {
    while (node_ != nodes_->end() && !node_->is_leaf) {
      ++node_;
    }
  }

ShapeTreeLeafIterator& operator++() {
    ++node_;
    while (node_ != nodes_->end() && !node_->is_leaf) {
      ++node_;
    }
    return *this;
  }
  ShapeTreeLeafIterator operator++(int) {
    auto i = *this;
    ++(*this);
    return i;
  }

ShapeTreeLeafIterator& operator--() {
    --node_;
    while (node_ > nodes_->begin() && !node_->is_leaf) {
      --node_;
    }
    return *this;
  }
  ShapeTreeLeafIterator operator--(int) {
    auto i = *this;
    --(*this);
    return i;
  }

bool operator==(const ShapeTreeLeafIterator& other) const {
    return node_ == other.node_;
  }
  bool operator!=(const ShapeTreeLeafIterator& other) const {
    return node_ != other.node_;
  }
  ValueType& operator*() const { return node_->data; }
  ValueType* operator->() const { return &node_->data; }

private:
  ContainerType* nodes_;
  IteratorType node_;
};

template <typename T>
int64 ShapeTree<T>::CountSubshapes(const Shape& shape) {
  int64 current_count = 1;
  if (shape.IsTuple()) {
    int64 count = ShapeUtil::TupleElementCount(shape);
    for (int i = 0; i < count; ++i) {
      current_count += CountSubshapes(shape.tuple_shapes(i));
    }
  }
  return current_count;
}

template <typename T>
void ShapeTree<T>::InitChildren(const Shape& shape, const T& init_value,
                                Node* node, Index* index) {
  if (shape.IsTuple()) {
    const int64 size = ShapeUtil::TupleElementCount(shape);
#ifndef NDEBUG
    index->children_count = size;
#endif
    node->is_leaf = false;
    ShapeIndex shape_index = node->data.first;
    shape_index.push_back(0);

// At the end of the index_table, reserve a continuous space to hold the
    // children of current node. In order to enforce the invariant that all
    // children of a given node are placed together, we need to do the
    // reservation before we recurse into any of its children.
    int64 children_start_position = index_table_.size();
    index_table_.resize(index_table_.size() + size);

for (int i = 0; i < size; ++i) {
      shape_index[shape_index.size() - 1] = i;
      index_table_[children_start_position + i].index = nodes_.size();
      // The first child of the node in the index table is placed at the end of
      // the table.
      index_table_[children_start_position + i].children_start =
          index_table_.size();
      nodes_.emplace_back(shape_index, init_value);
      InitChildren(shape.tuple_shapes(i), init_value, &nodes_.back(),
                   &index_table_[children_start_position + i]);
    }
  } else {
#ifndef NDEBUG
    index->children_count = 0;
#endif
  }
}

template <typename T>
void ShapeTree<T>::InitChildren(const Shape& shape, Node* node, Index* index) {
  if (shape.IsTuple()) {
    const int64 size = ShapeUtil::TupleElementCount(shape);
#ifndef NDEBUG
    index->children_count = size;
#endif
    node->is_leaf = false;
    ShapeIndex shape_index = node->data.first;
    shape_index.push_back(0);

for (int i = 0; i < size; ++i) {
      shape_index[shape_index.size() - 1] = i;
      index_table_[children_start_position + i].index = nodes_.size();
      // The first child of the node in the index table is placed at the end of
      // the table.
      index_table_[children_start_position + i].children_start =
          index_table_.size();
      nodes_.emplace_back(shape_index);
      InitChildren(shape.tuple_shapes(i), &nodes_.back(),
                   &index_table_[children_start_position + i]);
    }
  } else {
#ifndef NDEBUG
    index->children_count = 0;
#endif
  }
}

template <typename T>
ShapeTree<T>::ShapeTree(Shape shape)
    : shape_storage_(std::make_shared<Shape>(std::move(shape))),
      shape_(shape_storage_.get()) {
  const int64 count = CountSubshapes(*shape_);
  nodes_.reserve(count);
  nodes_.emplace_back(ShapeIndex{});

index_table_.reserve(count);
  index_table_.emplace_back(Index{0, 1});
  InitChildren(*shape_, &nodes_[0], &index_table_[0]);
}

template <typename T>
ShapeTree<T>::ShapeTree(const Shape* shape) : shape_(shape) {
  const int64 count = CountSubshapes(*shape_);
  nodes_.reserve(count);
  nodes_.emplace_back(ShapeIndex{});

index_table_.reserve(count);
  index_table_.emplace_back(Index{0, 1});
  InitChildren(*shape_, &nodes_[0], &index_table_[0]);
}

template <typename T>
ShapeTree<T>::ShapeTree(const std::shared_ptr<Shape>& shape)
    : shape_storage_(shape), shape_(shape_storage_.get()) {
  const int64 count = CountSubshapes(*shape_);
  nodes_.reserve(count);
  nodes_.emplace_back(ShapeIndex{});

index_table_.reserve(count);
  index_table_.emplace_back(Index{0, 1});
  InitChildren(*shape_, &nodes_[0], &index_table_[0]);
}

template <typename T>
ShapeTree<T>::ShapeTree(Shape shape, const T& init_value)
    : shape_storage_(std::make_shared<Shape>(std::move(shape))),
      shape_(shape_storage_.get()) {
  const int64 count = CountSubshapes(*shape_);
  nodes_.reserve(count);
  nodes_.emplace_back(ShapeIndex{}, init_value);

index_table_.reserve(count);
  index_table_.emplace_back(Index{0, 1});
  InitChildren(*shape_, init_value, &nodes_[0], &index_table_[0]);
}

template <typename T>
ShapeTree<T>::ShapeTree(const Shape* shape, const T& init_value)
    : shape_(shape) {
  const int64 count = CountSubshapes(*shape_);
  nodes_.reserve(count);
  nodes_.emplace_back(ShapeIndex{}, init_value);

index_table_.reserve(count);
  index_table_.emplace_back(Index{0, 1});
  InitChildren(*shape_, init_value, &nodes_[0], &index_table_[0]);
}

template <typename T>
ShapeTree<T>::ShapeTree(const std::shared_ptr<Shape>& shape,
                        const T& init_value)
    : shape_storage_(shape), shape_(shape_storage_.get()) {
  const int64 count = CountSubshapes(*shape_);
  nodes_.reserve(count);
  nodes_.emplace_back(ShapeIndex{}, init_value);

index_table_.reserve(count);
  index_table_.emplace_back(Index{0, 1});
  InitChildren(*shape_, init_value, &nodes_[0], &index_table_[0]);
}

template <typename T>
const T& ShapeTree<T>::element(ShapeIndexView index) const {
  return Lookup(index)->data.second;
}

template <typename T>
T* ShapeTree<T>::mutable_element(ShapeIndexView index) {
  return &Lookup(index)->data.second;
}

template <typename T>
internal::ShapeTreeNode<T>* ShapeTree<T>::Lookup(ShapeIndexView index) {
  Index* iter = &index_table_[0];
  for (const int64 i : index) {
    CHECK_GE(i, 0);
#ifndef NDEBUG
    CHECK_LT(i, iter->children_count);
#endif
    iter = &index_table_[iter->children_start + i];
  }

return &nodes_[iter->index];
}

template <typename T>
const internal::ShapeTreeNode<T>* ShapeTree<T>::Lookup(
    ShapeIndexView index) const {
  return const_cast<ShapeTree*>(this)->Lookup(index);
}

/* static */
template <typename T>
template <typename Fn>
Status ShapeTree<T>::ForEachHelper(const Fn& func,
                                   const std::vector<Node>& nodes) {
  for (const auto& node : nodes) {
    TF_RETURN_IF_ERROR(func(node.data.first, node.data.second));
  }
  return Status::OK();
}

/* static */
template <typename T>
template <typename Fn>
Status ShapeTree<T>::ForEachMutableHelper(const Fn& func,
                                          std::vector<Node>* nodes) {
  for (auto& node : *nodes) {
    TF_RETURN_IF_ERROR(func(node.data.first, &node.data.second));
  }
  return Status::OK();
}

template <typename T>
template <typename Fn>
Status ShapeTree<T>::ForEachElementWithStatus(const Fn& func) const {
  return ForEachHelper(func, nodes_);
}

template <typename T>
template <typename Fn>
Status ShapeTree<T>::ForEachMutableElementWithStatus(const Fn& func) {
  return ForEachMutableHelper(func, &nodes_);
}

template <typename T>
template <typename Fn>
void ShapeTree<T>::ForEachElement(const Fn& func) const {
  return ForEachHelper(
             [&func](const ShapeIndex& index, const T& data) {
               func(index, data);
               return Status::OK();
             },
             nodes_)
      .IgnoreError();
}

template <typename T>
template <typename Fn>
void ShapeTree<T>::ForEachMutableElement(const Fn& func) {
  return ForEachMutableHelper(
             [&func](const ShapeIndex& index, T* data) {
               func(index, data);
               return Status::OK();
             },
             &nodes_)
      .IgnoreError();
}

template <typename T>
void ShapeTree<T>::CopySubtreeFrom(const ShapeTree<T>& other,
                                   const ShapeIndex& source_base_index,
                                   const ShapeIndex& target_base_index) {
  CHECK(ShapeUtil::Compatible(
      ShapeUtil::GetSubshape(shape(), target_base_index),
      ShapeUtil::GetSubshape(other.shape(), source_base_index)))
      << ShapeUtil::GetSubshape(shape(), target_base_index) << " vs "
      << ShapeUtil::GetSubshape(other.shape(), source_base_index);
  ForEachMutableElement([this, &other, &source_base_index, &target_base_index](
                            const ShapeIndex& index, T* data) {
    // Copy the data element only if index is in the
    // subtree rooted at target_base_index.
    for (int i = 0; i < target_base_index.size(); ++i) {
      if (i >= index.size() || index[i] != target_base_index[i]) {
        return;
      }
    }
    // Construct source element index to copy from.
    ShapeIndex source_index = source_base_index;
    for (int i = target_base_index.size(); i < index.size(); ++i) {
      source_index.push_back(index[i]);
    }
    *data = other.element(source_index);
  });
}

template <typename T>
StatusOr<ShapeTree<T>> ShapeTree<T>::SubShapeTree(
    const ShapeIndex& index) const {
  TF_ASSIGN_OR_RETURN(const Shape* sub_shape,
                      ShapeUtil::TryGetSubshape(shape(), index));
  ShapeTree<T> sub_shape_tree(*sub_shape);
  sub_shape_tree.CopySubtreeFrom(*this, index, {});
  return std::move(sub_shape_tree);
}

template <typename T>
bool ShapeTree<T>::operator==(const ShapeTree<T>& other) const {
  bool equal = true;
  ForEachElement([&other, &equal](const ShapeIndex& index, const T& data) {
    if (data != other.element(index)) {
      equal = false;
    }
  });
  return equal;
}

}  // namespace xla

#endif  // TENSORFLOW_COMPILER_XLA_SHAPE_TREE_H_